The present invention relates to a process for preparing a polyimide film containing small amounts of an organometallic compound, which when cured at a high temperature, provides a film with improved cohesive strength thereby providing better adhesion of the film to a metal foil such as copper, silver, gold or indium.
Polyimide films are well-known for their outstanding properties such as heat resistance, low temperature resistance, chemical resistance, electrical insulation and mechanical strength. Such polyimide films are widely used as electrical insulating films and as flexible printed circuit substrates. In such applications, the polyimide films are usually bonded to a copper foil with an acrylic or epoxy adhesive and, therefore, good cohesive strength in the film and good adhesion are important properties.
U.S. Pat. No. 3,389,111, issued to McKeown et al. on Jun. 18, 1968, discloses a polyimide composition having improved corona resistance containing from 0.1 to 20% by weight of an organometallic compound of a metal selected from Group IVb or Vb of the Periodic Table.
In the present invention, a very small amount of an organometallic compound is incorporated into the polyimide film to improve the cohesive strength of the polyimide, after high temperature curing, thereby improving adhesion. The prior art differs in using much larger amounts of an organometallic compound to improve corona resistance of the polyimide film and does not achieve the desired improvement in cohesive strength by high-temperature curing.
It is an object of the present invention to provide a polyimide film having a structure with improved cohesive properties and which has improved adhesion. It is a further object of the invention to provide a strengthened polyimide film having improved adhesive properties when bonded to metal foils, such as copper, silver, gold or indium, using an epoxy, acrylic or other heat resistant adhesive.